Always load preview when in preview mode, not in edit mode
API New CMSForm class to allow validation responses in CMS (fixes #1777)

Thanks to @willmorgan for getting this discussion started
(see https://github.com/silverstripe/sapphire/pull/1814).
Consistent Form setters (returning $this on setHTMLID())
Merge pull request #2084 from chillu/pulls/cmsform

Handle ValidationException on CMS forms